We continued to utilize two independent, recently developed, transgenic in vivo mutagenicity assays to study the role of B-cell mutagenesis in the development of inflammation-induced BALB/c plasmacytomas. The assays are based on the phage l-derived shuttle vector, lLIZ, or the plasmid-derived shuttle vector, placZ. They are aimed at elucidating (a) the putative mutagenicity of the inflammatory granuloma (i.e., the tissue site of plasmacytoma development), (b) the genetic control of B-cell mutagenesis (on a plasmacytoma-susceptible background as opposed to a plasmacytoma-resistant background), and (c) the importance of oxidative mechanisms in mutagenizing B cells. To devise an experimental system that is conducive for the above-mentioned aims, Klaus Felix decided to backcross the two shuttle vectors, lLIZ and placZ, onto plasmacytoma-susceptible BALB/cAnPt mice and plasmacytoma-resistant DBA/2N mice. He has completed these backcrosses in this report period, and he will thus be able to utilize from hereon genetically pure, congenic BALB/cAnPt and DBA/2N mice harboring lLIZ or placZ. Using incompletely backcrossed BALB/cAnPt.lLIZ mice (at generation N5) in a pilot experiment designed to prove the principal feasibility of our approach, Klaus Felix found that elevated mutant rates persisted for surprisingly long periods of time in lymphoid tissues of mice exposed to sustained oxidative stress (Cancer Res. 59, 3621-3626, 1999). This result proved feasibility and added confidence to our overall experimental strategy. Collaborating with Lynne Rockwood, Klaus Felix also initiated studies on the relationship between general cellular metabolism and mutation control. Using the partial loss-of-function mutant of a house-keeping gene, glucose 6-phosphate dehydrogenase, he was able to demonstrate that the diminished ability of B cells to produce reducing equivalents (in form of NADPH2) is associated with elevated levels of spontaneous mutagenesis in vivo. Genomic instability can thus be caused by weak alleles in metabolic pathways not implied previously in mutation control. Ming Qian, who joined the program in this fiscal year, has chosen to explore the hypothesis that the deregulated expression of the proto-oncogene c-myc, a hallmark of BALB/c plasmacytomas, may result in elevated mutagenesis and genomic instability. He is currently developing methods for comparing the inducible expression of c-myc with mutagenesis in vivo.